Conventionally, as described in JP-A-2002-98712, a capacitive physical quantity detector includes a sensor element having a capacitance, which is changeable according to a physical quantity, and a C-V converting circuit for converting the capacitance change of the sensor element to a voltage. The C-V converting circuit includes an operational amplifier, a switching element disposed between an input terminal and an output terminal of the operational amplifier, multiple feedback capacitors, and a selector for selecting a feedback capacitor connecting in parallel to the switching element according to an external signal. The detection range of the physical quantity such as acceleration in the capacitive physical quantity detector can be changeable according to the external signal.
The capacitance change of the sensor element depends on the acceleration. The capacitance change provides to accumulate charges in the feedback capacitor. The voltage of the signal output from the C-V converting circuit is in proportion to the charges accumulated in the feedback capacitor, and in reverse proportion to an electrostatic capacitance of the feedback capacitor. Accordingly, when the applied acceleration is small, and the electrostatic capacitance of the feedback capacitor is large, the voltage of the output signal from the C-V converting circuit is small. Thus, it is difficult to detect the acceleration. On the other hand, when the applied acceleration is large, and the electrostatic capacitance of the feedback capacitor is small, the voltage of the output signal from the C-V converting circuit may be saturated, so that the acceleration is not detected accurately.
Thus, the detection range of the acceleration depends on the electrostatic capacitance of the feedback capacitor. When the large acceleration is detected, the feedback capacitor having the large electrostatic capacitance is connected in parallel to the switching element. When the small acceleration is detected, the feedback capacitor having the small electrostatic capacitance is connected in parallel to the switching element. Thus, the acceleration in a certain range is detected. Here, in a detection range of the acceleration, in which the voltage of the output signal from the C-V converting circuit is not saturated, when the feedback capacitor having much small electrostatic capacitance is selected, the voltage of the output signal from the C-V converting circuit is large, so that the acceleration is detected with high sensitivity.
In the capacitive physical quantity detector described in JP-A-2002-98712, the feedback capacitor is selected according to the external signal so that the detection range of the acceleration is adjusted. Accordingly, when the acceleration beyond the detection range of the feedback capacitor adjusted based on the external signal is applied, the detector cannot detect the acceleration. Further, in some cases, the feedback capacitor may not be selected for detecting the acceleration with high sensitivity, so that the detector cannot detect the acceleration with high sensitivity.
The output signal of the C-V converting circuit is input into an external device via a sample holding circuit, a low pass filter and an amplifier circuit. The external signal is input into a decoder as a main element of a selecting circuit. Based on the output signal from the decoder, the feedback capacitor connecting in parallel to the switching element is selected. In this case, for example, it is considered that the external signal to be input into the decoder is generated according to the signal output from the amplifier circuit to the external device. Further, the generated external signal is input into the decoder so that the feedback capacitor is selected such that the feedback capacitor is suitable for detecting the acceleration with high sensitivity and for detecting the acceleration within the detection range.
However, the output signal of the C-V converting circuit is input into the external device via a processing element including the low pass filter and disposed on an output side of the C-V converting circuit. Accordingly, the output signal is delayed by the processing element on the output side of the C-V converting circuit and an element in the external device of the detector. As a result, it is difficult to select the feedback capacitor having the detection range of detecting the acceleration in quick response to the applied acceleration.